K. K. VON BAER. PHILOSOPHICAL FRAGMENTS. 199 



nates, although in each of the three segments none of these is 

 wholly absent. 



Together with the principal contrast of anterior and posterior, 

 a slighter one of upper and lower is to be recognized in the 

 higher stages of development. A distinction into right and left 

 sides is only exceptionally present, and as a rule is absent. A 

 perpendicular plane therefore divides the body into two equal 

 halves, whence this organization may be called symmetrical. 

 Every single organ lies in this middle plane, so long as the 

 shortening of the whole body does not drive the less shortened 

 intestine out of the median plane*; and every organ which lies 

 in the median plane is single, while that which lies external to 

 it is double. In the median plane, however, we must recognize 

 a difference of above and below ; the animal organs are concen- 

 trated below, the plastic ones above, if we regard only the in- 

 ternal structure. The trunks of the animal organs also lie below, 

 and they form here a kind of axis, from which the whole organi- 

 zation proceeds on both sides upwards, a relation which is ren- 

 dered obvious by the mode of development (compare Coroll. 4). 



The sensible and the irritable functions are especially deve- 

 loped in this series of animals; their movements are lively, and 

 are the more distinctly directed forwards the more the body is 

 drawn out longitudinally. When the body is shortened, as in 

 the Spiders and Crabs, so much the less determinate is the 

 direction of their movements ; the plastic organs are less deve- 

 loped, the glands are particularly rare, and are generally re- 

 placed by simple tubes. 



I believe that a third type is recognizable in the Mollusca ; 

 and as belonging to its lower grades of development I enume- 

 rate the Rotifera and such Infusoria as are coiled, or at least 

 are neither symmetrical nor peripheral. This type may be called 

 the massive type, for neither length nor surface predominates, 

 but the whole body and all its parts are formed in more rounded 

 masses, which are either solid or hollow. 



As the principal antithesis of animal organization, that of in- 

 gestion and of egestion, is manifested neither by the two opposite 



* In fact, it appears that in Insects which possess metamorphosis, the in- 

 testine is moved out of the median plane, sometimes only in the last stage of 

 metamorphosis, sometimes even in the larval condition 



